Hydrocarbon wells are typically completed with an assembly of concentric steel pipes, referred to as conductors, casing, or tubing. The completion extends from a wellhead downwardly into the wellbore. The wellhead can be at land, or in offshore installations at a platform. Several parts of the completion can be subject to corrosion. In offshore installations for example, sea water can be present in an outer annulus of a completion, such as between a conductor and the outer casing. Corrosion sensitive areas are at sea level, where tidal effects and waves cause exposure of the well completion to water and air, or in a so-called splash zone even to aerated water. There is a need to perform inspection of such completions.
Inspection of well casing by passing a tool centrally down a tubular or casing is known from e.g. specification U.S. Pat. No. 4,701,892. Such inspection tools interfere with production of hydrocarbons, however. If it is known beforehand that corrosion inspection is desired, a fixed downhole corrosion monitoring apparatus comprising piezoelectric transducers as known from U.S. Pat. No. 6,131,659 can be installed. If fixed equipment is not present and if production is not to be disturbed, oftentimes the only practical access path is through the open annulus between tubulars.
An option in this case is visual inspection by passing video equipment down the annulus, but there is a need for a more precise inspection method that can spot anomalies that can compromise the integrity of steel objects, e.g. a significant wall thickness decrease with respect to a norm. Apart from well completions, similar problems are also encountered in other situations where two metal objects, in particular steel objects have to be inspected from a narrow intermediate space between them.